The primary objective of this research is to determine the effects of aging on the sexual behavior of male rhesus monkeys. Behavior will be correlated with plasma levels of gonadal hormones. Sexual activity will be studied in standardized pair tests with receptive females; hormone levels will be determined by means of radioimmunoassays. Attempts to influence the level of sexual performance will be made by manipulation of the test conditions and hormone levels. Co-workers and I will continue: 1. to study the decline in sexual behavior of intact and long-term (9 years) castrated males, 2. to determine the effectiveness of androgens in restoring the behavior, and 3. to investigate changes in sensitivity of the negative feedback mechanism. The sexual behavior of these males has been studied for over 10 years. Extensive behavioral and hormonal data and complete health records make them a unique population on which to carry out the proposed research.